When employed as the light source for recording data into or reading data out from a recording medium, short-wavelength laser light has the advantage of enabling an increased recording density. In addition, short-wavelength laser light is also advantageous when employed in material processing applications, as its heat effects are small and it makes precision processing possible. Short-wavelength laser light is also being used such as a light source in the medical field, and a lithography light source for a very large-scale integrated circuit.
Thus, short-wavelength laser light is desired in many diverse fields. Accordingly, there has been a demand for a small, lightweight, long-lasting light source that stably radiates short-wavelength laser light.
However, a suitable light source that radiates light having a wavelength of 500 nm or less has not been conventionally available. For example, while semiconductor lasers are known that can radiate laser light having wavelengths of up to 400 nm, these devices have been problematic because of their extremely low output.
Excimer lasers are available as examples of short-wavelength large-output lasers. These lasers were first realized in 1970 by Basov et al in the former Soviet Union using a method of exciting liquid xenon (Xe) with an electronic beam. In 1976, these lasers were successfully oscillated using electric discharge pumping. In excimer lasers of this type, i.e., employing electric discharge pumping, ultraviolet light is generated by compounds such as ArF (193 nm), KrF (248 nm), or XeCl (308 nm) in an ultraviolet pulse repetition oscillating laser, amplified using an optical resonator, and then output as laser light. Application of excimer lasers has been much anticipated in fields such as polymer ablation, surface reforming, marking, thin film formation, medical product manufacturing, and isotope separation. However, when pulse lasers which repeatedly generate several hundred pulses per second are used as excimer lasers, they can only generate a 10−9 second pulse light every 10−2 seconds. That is, the duration during which the laser is being generated is extremely short compared to the interval, so that application of excimer lasers in a deposition process or the processing steps employed in the aforementioned fields is problematic. Furthermore, excimer lasers are also problematic with respect to the short lifespan of the gas medium, difficulty in reducing the size of the laser device, poor maintenance, high operational costs, employment of toxic gases, etc. Thus, the practical utilization of semiconductor lasers, etc. that can generate light in the ultraviolet region at room temperature, stably and over a long period of time, has yet to be realized.
There has therefore been increased research activity in recent years in the area of nonlinear optical elements such as second harmonic-wave generating (SHG) elements. SHG elements generate light having one-half the wavelength of the incident light so that, for example, light in the ultraviolet region can be generated using laser light in the infrared region. Thus, the industrial value of this technology in various fields of application is extremely large.
Conventionally known crystals employed as wavelength converting elements like SHG elements include KTP (KTiOPO4) disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 3-65597, and BBO (β-BaB2O4), CLBO (CsLiB6O10), LBO (LiB3O5), and KDP (KH2PO4), etc. disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 63-279231.
However, in the case of a wavelength converting element employing KTP, not only is it difficult to increase the size of the crystal, but the refractive index varies inside the crystal. Accordingly, even in the case of KTP elements that are cut from a single crystal, the refractive indices will differ from one another. As a result, the phase matching angles differ, making it difficult to realize a wavelength converting element that is highly precise. Further, since pores are readily generated in a KTP type crystal, it is difficult to supply a large amount of high-quality KTP crystals.
In addition, while converting elements employing BBO or CLBO have high conversion efficiency, they are problematic with respect to resistance to moisture and laser damage, and output destabilization due to two photon absorption.
In converting elements employing LBO, the shortest SHG wavelength (second harmonic wave) is 277 nm, so that the wavelength conversion range is narrow. For this reason, these devices cannot generate the fourth harmonic wave (266 nm) of an Nd:YAG laser. Further, another disadvantage is that a large crystal is not possible.
In converting elements employing KDP, phase mismatching arises due to the effects of heat absorbed at a high repetition rate. Accordingly, these elements cannot be used unless a low repetition rate of 100 Hz or less is employed. In addition, at a high repetition rate, the threshold for damage is extremely low. Accordingly, it is difficult to employ this device in laser oscillators used in manufacturing or industrial applications that are employed at repetition rates exceeding 1 kHz.
The present applicant therefore previously proposed a wavelength converting method employing an LB4 (Li2B4O7) single crystal as a converting element (Japanese Patent Application No. Hei 8-250523).
This LB4 single crystal is highly transmissive with respect to a wide range of wavelengths and incurs little damage from the laser light. Further, a large crystal with excellent quality can be manufactured easily. In addition, this LB4 single crystal is superior with respect to workability, low deliquescence, and excellent ease of handling. In addition, this crystal has a long lifespan.
Accordingly, a small, lightweight, inexpensive optical converting element can be realized using LB4 that can be operated stably over a long period of time, has a long lifespan, and excellent workability.
The conversion efficiency of a wavelength converting element is determined mainly by the inherent physical properties of the crystal, such as its nonlinear optical constant and the tolerance zone for the phase matching angle. An LB4 single crystal has the disadvantage of low conversion efficiency when compared to BBO and CLBO. For this reason, it was felt that an LB4 single crystal with its low conversion efficiency was not suitable for use as a wavelength converting element for radiating light in the ultraviolet region.
In order to improve the low conversion efficiency and obtain radiated light of a high average output, a variety of technical methods can be employed. Conventionally employed methods include, for example, increasing the peak power density of the incident light by using a lens to converge the incident light; increasing the crystal length; using a plurality of wavelength converting crystals; and employing as the light source a laser oscillator that has high quality beam characteristics, i.e., little beam spreading at high outputs.
However, improving the conversion efficiency using these types of technical methods has had the following limitations.
First, in the method for increasing the peak power density of the incident light by converging the incident light with a lens, the peak power density cannot be increased limitlessly; rather, consideration must be given to laser damage from the incident light.
In other words, an antireflection film to reduce reflection is typically coated onto the end face of the crystal element in the wavelength converting element. However, in general, this antireflection film's resistance to damage by the laser is not all that sufficient, so that damage can be incurred if the peak power density of the incident light is high. In addition, when the light is input at a high peak power density, it is possible for the crystal element itself to suffer dielectric breakdown. Accordingly, the wavelength converting element's laser damage threshold, including the characteristics of the antireflection film, must be taken into consideration, and appropriate limits then applied to the peak power density of the incident light.
In addition, even in the case where high conversion efficiency is obtained by increasing the peak power density of the incident light, nonlinear optical crystals have the unique problem of two photon absorption. This is a phenomenon whereby, as a result of two photon absorption by the crystal itself, a donut-shaped hole opens up in the center of the radiated light beam pattern, leading to extremely unstable output. Two photon absorption can strengthen in proportion to the square of the beam intensity of the radiated light. Thus, heating within the crystal from absorption can have a large effect, particularly at the high intensity beam center, causing the refractive index to vary and disrupting phase matching.
Note that for the purpose of protecting nonlinear crystals from moisture, or to perform phase matching using temperature, it has been the conventional practice to heat and maintain nonlinear optical crystals at 40–200° C.
When a lens is used to converge incident light, spreading of the incident beam increases. As a result, the tolerance zone for the phase matching angle is exceeded, and conversion efficiency decreases.
In the case of the method in which crystal length is increased, the tolerance zone for the phase matching angle narrows and absorption by the crystal increases when the crystal is made longer. Once a specific length has been exceeded, there is a tendency for the conversion efficiency to gradually become saturated. In addition, strain arises in the beam pattern from walk-off when the crystal becomes longer. Thus, this crystal lengthening approach, as well, cannot be deemed entirely effective.
In the method employing a plurality of individual wavelength converting crystals, a beam passes through a crystal without undergoing wavelength conversion is reused by being made to input to the next crystal. In this method, not only does the conversion efficiency increase, but an increased output may be expected from the effects of interference between wavelength converted light generated by the plurality of individual crystals. However, when there is broad spreading of the incident light beam, or when the beam diameter is small in this method, it is not possible to obtain a sufficient interference effect.
In the case of the method in which a laser oscillator having high quality beam characteristics is employed as the light source, use of a beam that experiences little spreading at high power is certainly ideal from the perspective of increasing conversion efficiency. However, it is difficult to make this type of oscillator at low cost.
Further, as an additional problem, as explained above, while use of various nonlinear optical crystals as converting elements is known, a method has not yet been achieved that enables second harmonic waves and other such high power sum frequency waves to be obtained with good efficiency.
In other words, in order to obtain high power sum frequency waves like second harmonic waves, it is first necessary to employ a converting element that can achieve a high conversion efficiency. Secondly, in order to enable conversion of high power incident light, it is necessary to use a converting element that possesses high resistance to damage from the incident light.
However, it is generally the case that nonlinear optical crystals that have high conversion efficiency have poor resistance to damage, while nonlinear optical crystals that are highly resistant to damage have poor conversion efficiency. Thus, a nonlinear crystal equipped with both sufficient conversion efficiency and resistance to damage has not been available.
The present invention was conceived in view of the above-described problems and is directed to the provision of an optical wavelength converting method, and to an optical wavelength converting system, program and medium, which enable production of an all solid state ultraviolet laser oscillator that stably achieves a high conversion efficiency using a nonlinear optical crystal, lithium tetraborate single crystal LB4 for example, and is durable with respect to practical applications (first problem).
The present invention is further directed to the provision of an optical wavelength converting method, an optical wavelength converting system, and a laser oscillating system that compensate for the restrictive conditions of the nonlinear optical crystals that can be employed and are capable of obtaining high power sum frequency waves such as second harmonic waves with good efficiency (second problem).